Length sensor

ABSTRACT

A length measuring probe that includes a base body, a guide element and a probe pin having a touch scanning element for touch scanning a measuring object, the probe pin is seated, displaceable in relation to the base body in a measuring direction opposite a spring force F via the guide element. A detection device detects a position of the probe pin with respect to the base body and a first spring element and a second spring element arranged behind the first spring element in the measuring direction so as to prestress the probe pin. The guide element is arranged between the first and second spring elements, on which facing end areas of the first and second spring elements are fixed in place, and wherein the at least one guide element is seated, displaceable in the measuring direction, on the probe pin and on the base body.  
     The invention relates to a length sensor with a relatively large measuring zone, that is with a large lift of the stylus ( 2 ), wherein a plurality of helical springs ( 7, 8 ) are arranged one behind the other in order to pretension the stylus ( 2 ). A ball bearing arrangement ( 4 ) is interposed between two helical springs ( 7, 8 ) and bears the stylus ( 2 ) in such a manner that it can be displaced in the longitudinal direction X and is mounted itself on the base ( 1 ) so as to be displaceable in the longitudinal direction X.

[0001] The invention relates to a length measuring probe in accordance with the preamble of claim 1.

[0002] Such length measuring probes are employed in manufacturing technology for measuring workpieces. For example, the thickness, width, length, depth, as well as the inner or outer diameters can be determined.

[0003] A length measuring probe with the characteristics of the preamble of the claim is described in U.S. Pat. No. 5,072,174. A probe pin is seated in a base body by means of a guide element of a ball bearing guide, so that it is displaceable in the longitudinal direction. The contact pressure required for scanning a workpiece to be measured by touch is generated by a helical spring, whose one end is fixed on the probe pin and the other end on the base body.

[0004] The length of the helical spring must correspond at least to the lift of the probe pin, however, long helical springs have a tendency to kink laterally. The greater the desired lift of the probe pin, the longer the ball bearing guide must be embodied in order to be able to absorb the lateral forces introduced by the probe pin in the course of touch scanning because of the lever effect. Therefore length measuring probes with a relatively large measuring range—i.e. lift of the probe pin—are constructed to be relatively long.

[0005] For shortening the structural length of a length measuring probe, two ball bearing guides are provided in accordance with U.S. Pat. No. 4,347,492, which are spaced apart from each other. A detection device for detecting the position of the probe pin is arranged between these two ball bearing guides. The helical spring is located in a bore in the probe pin, which requires a relatively thick probe pin. In the case where a large lift of the probe pin is required and a long helical spring is used, the problem of kinking also occurs here, which causes friction between the helical spring and the probe pin.

[0006] It is the object of the invention to create a length measuring probe which is constructed to be compact and solid, even with a relatively large lift of the probe pin, or wherein the guide element does not adversely affect the measuring process.

[0007] This object is attained by means of the characteristics of claim 1.

[0008] The advantages of the invention lie in that the helical springs are arranged in a space-saving manner, and the probe pin is seated over the entire lifting distance in a manner in which the transverse forces are stabilized. Kinking of the helical springs is prevented by dividing a long helical spring into several short helical springs with guide elements arranged between them. There is now the possibility of employing helical springs which generate a small pressure or tensional force F and do not kink in spite of their thin walls and the therefore small spring constant. By means of this an interfering friction between the probe pin and the windings of the helical springs, which would affect the service life and accuracy of measurement of the helical springs, is prevented. It is now possible to select the arrangement in such a way that during the measuring operation the scale fastened to the probe pin dips into the windings of at least one of the helical springs.

[0009] A further advantage which is obtained by the invention is the defined movement of the guide element. The helical springs always maintain the guide element in a defined manner between the springs, in case of equal spring properties in the center between the springs because of the equilibrium of the forces.

[0010] Advantageous embodiments of the invention are recited in the dependent claims.

[0011] Exemplary embodiments will be explained in greater detail in the subsequent description in connection with the drawings.

[0012] Shown are in:

[0013]FIG. 1, a sectional view of a length measuring probe in an extended end position of the probe pin,

[0014]FIG. 2, the length measuring probe in accordance with FIG. 1 in a touch scanning position,

[0015]FIG. 3, a portion of a length measuring probe with a tension spring fixed in place in it, and

[0016]FIG. 4, a sectional view of a further length measuring probe.

[0017] An exemplary embodiment of a length measuring probe designed in accordance with the invention will be described in what follows by means of FIGS. 1 and 2. The drawing figures schematically show a cross section of the entire length measuring probe, consisting of a base body 1 in which a rod-shaped probe pin 2 is seated, displaceable in the measuring direction X.

[0018] For touch scanning a workpiece 5 represented in FIG. 2, the end of the probe pin 2 is spherically designed as a touch scanning element 6. In the course of a touch scanning process, the touch scanning element 6 of the probe pin 2 is forced with a force F onto the surface of the workpiece 5 to be measured. This force F is generated by helical springs 7 and 8, which are arranged one behind the other in the measuring direction X and force the touch scanning element 6 onto the workpiece 5. For a space-saving construction, the windings of the helical springs 7, 8 extend around the circumference of the round probe pin 2. For preventing friction between the probe pin 2 and the helical springs 7, 8 in the course of the touch scanning movements of the probe pin 2, a free space between the probe pin 2 and the windings of the helical springs 7, 8 is provided over the circumference.

[0019] A detection device 9 is provided for measuring the linear displacement of the probe pin 2 with respect to the base body 1. In a known manner it consists of a scale 91 and a scanning unit 92 for scanning the measurement graduation of the scale 91. The scale 91 is fastened on the probe pin 2, and the scanning unit 92 opposite the latter on the base body 1. Position-dependent electrical signal are generated by the scanning unit 92 and are supplied to a display unit or an evaluation unit. The measurement graduation of the scale 91 can be embodied in such a way that it can be scanned opto-electrically, capacitively, magnetically or inductively. For a particularly space-saving construction, the scale 91 has such dimensions that it can dip into the windings of the helical spring 8 during lifting movements of the probe pin 2. To this end, the axial distance between the stationary scanning unit 92 and the rear end of the helical spring 8 is less than the possible lift of the probe pin 2, or than the measuring range of the detection device 9.

[0020] Guidance of the probe pin 2 in the base body 1 is provided by means of several guide elements, which are arranged one behind the other in the measuring direction X and are in particular designed as ball bearing guides 3 and 4. One of these ball bearing guides 3 is arranged at the end of the base body 1 toward the touch scanning side. In order to be able to absorb the transverse forces introduced in the course of touch scanning by the probe pin 2 well, at least one further ball bearing guide 4 is arranged at a distance from it. For a particularly space-saving construction, this further ball bearing guide 4 is arranged between the two helical springs 7 and 8. In this case the first helical spring 7 is supported at one end on the probe pin 2, and with the other end on the ball bearing guide 4. In the same way one end of the helical spring 8 is supported on the opposite side of this ball bearing guide 4, while the other end of this helical spring 8 is supported on the base body 1.

[0021] Each one of these ball bearing guides 3 and 4 consists of a ball bearing cage 31 and 41, with balls 32 and 42 rotatably seated therein, which on the one side roll off on the inner surface of the base body 1 and on the other side on the surface of the probe pin. Since the helical springs 7 and 8 are compression springs, they are fixed in place on the ball bearing cages 31 and 41 by being axially supported in the measuring direction X on the ball bearing cages 31, 41. In the support area, the ball bearing cages 31, 41 have a cup-shaped section 33, 43, 44, which extends around the helical springs 7, 8.

[0022] The ball bearing guide 4 is axially clamped between the two helical springs 7 and 8 and is movable in the measuring direction X in relation to the probe pin 2, as well as the base body 1, but transversely to this is seated without play on the probe pin 2 and on the base body 1. In the course of lifting movements of the probe pin 2 in the measuring direction X, the position of the ball bearing guide 4 is matched to the position of the helical springs 7, 8, it is always in the center position between the two helical springs 7, 8, provided these have the same spring constant.

[0023] The arrangement represented in FIGS. 1 and 2 has the advantage that relatively short ball bearing guides 3, 4 are sufficient for supporting transverse movements of the probe pin 2 well. In comparison to a single long helical spring, kinking of the helical springs 7, 8 is prevented by the ball bearing guide 4 between the two helical springs 7 and 8, since their ends are well guided by the ball bearing guide 4.

[0024] It is particularly advantageous if the base body 1 is designed in one piece over the entire range of all ball bearing guides 3 and 4 in order to prevent alignment errors of the guide surfaces on the base body 1.

[0025] Other guide elements, for example sliding bearings, can also be employed instead of the ball bearing guides 3 and 4. In the course of this it is also possible to replace the ball bearing guide 3 on the touch scanning side by a slide bearing fixed in place on the base body 1, or by a circulating ball bearing guide fixed in place on the base body 1. However, if the ball bearing guide 4 is replaced by a sliding bearing, the latter must have an outer sliding face between the bearing body and the base body 1, as well as an inner sliding face between the bearing body and the probe pin 2, in order to achieve the free mobility of the bearing body in relation to the base body 1, as well as the probe pin 2, in the axial direction X, as well as freedom from play transversely to it.

[0026] The guide element in the form of a ball bearing guide 4 or of a bearing body of a sliding bearing clamped between the two helical springs 7, 8 follows the lifting movement of the probe pin 2 by matching its position to the position of the helical springs 7, 8. When using two helical springs 7, 8, each helical spring 7, 8 is compressed by one half the lift movement of the probe pin 2. It is possible to also arrange further helical springs behind each other in addition to the two helical springs 7, 8, wherein advantageously a guide element, in particular a ball bearing guide 4, is arranged between two of these helical springs.

[0027] If the ball bearing guide 4 between the two helical springs 7, 8 is designed to be appropriately transversely stable, the probe pin 2 can also be exclusively guided by this ball bearing guide 4, and the ball bearing guide 3 on the touch scanning side can be omitted, which will be explained later by means of an example.

[0028] For automating the touch scanning procedure it is possible to provide a compressed air connector on the length measuring probe for generating a force which acts counter to the touch scanning force F of the compression springs 7, 8 and lifts the probe pin 2 off the workpiece 5.

[0029] The helical springs 7 and 8 can also be embodied as tension springs instead of as compression springs. By means of the tension springs, the probe pin 2 is in its position of rest in the retracted state. The touch scanning force F is pneumatically generated by means of compressed air in a known manner in that the compressed air is conducted into the interior of the length measuring probe and by means of this a linear movement of the probe pin 2 is initiated. If the touch scanning is to be terminated in an automated way, the compressed air is shut off with the result that the probe pin 2 is lifted off the workpiece by the tension springs and is retracted back into its position of rest. When using tension springs in place of compression springs 7, 8, the ends of the tension springs are fixed in place on the probe pin 2, the base body 1 and the guide elements 3, 4. This fixation in place can be provided by gluing, welding or a positive connection. A positive connection is particularly advantageous, an example of this is represented in FIG. 3. FIG. 3 shows the fixation in place of one end of a helical tension spring 10 on the probe pin 2. For fixation in place a sleeve 11 has been pressed on the circumference of the probe pin 2. This sleeve 11 has a collar 12 extending over a portion of its circumference, under which the end of the tension spring 10 has been screwed. The collar 12 can extend level, i.e. is produced by a groove in the sleeve 11, or the collar 12 can extend helically on the circumference of the sleeve 11 with a lead in the longitudinal direction X.

[0030]FIG. 4 represents a further exemplary embodiment of a length measuring probe. Different from the embodiments in accordance with FIGS. 1 and 2, no ball bearing guide 3 is provided here on the touch scanning side. The probe pin 2 is prestressed by the two helical springs 7 and 8, or is maintained in a position of rest and is prestressed during a touch scanning movement. The ball bearing guide 4 is arranged between the two helical springs 7, 8, and the ends of the helical springs 7, 8 associated with it are supported thereon (in the case of compression springs), or—as represented—they are fixed thereon by being fastened when using tension springs. For this fastening, end areas of the ball bearing cage 4 can each have a collar 12, identified in FIG. 3, for the positive hooking of the helical springs 7, 8. Moreover, one of the helical springs 7 is supported on the base body 1, and the other helical spring 8 on the probe pin 2. Advantageously the two helical springs 7, 8 have the same spring properties, so that the ball bearing guide 4 is centered between the support point on the base body and the support point at the probe body 2 in every position of the probe body 2 with respect to the base body 1.

[0031] With all embodiments, a permanent displacement of the ball bearing guide 4, caused by acceleration, blows or guidance inaccuracies, is prevented. The two helical springs, embodied as compression or tension springs, assure that any undesired displacement of the ball bearing guide 4 in the measuring direction X is compensated by restoring forces of the helical springs 7, 8. Because of this, the capability of the length measuring probe to function is assured, because the ball bearing guide 4 cannot be displaced into any end position, which would cause the braking of the probe pin movement. 

1. A length measuring probe, having a probe pin (2) with a touch scanning element (6) for touch scanning a measuring object (5), which is seated, displaceable in relation to a base body (1) in the measuring direction X opposite a spring force F, by means of at least one guide element (4), a detection device (9) for detecting the position of the probe pin (2) with respect to the base body (1), characterized in that the probe pin (2) is prestressed by several spring elements (7, 8) arranged one behind the other in the measuring direction X, a guide element (4) is arranged between respectively two spring elements (7, 8), on which the facing end areas of these two spring elements (7, 8) are fixed in place, wherein the guide element (4) is seated, displaceable in the measuring direction X, on the probe pin (2) and on the base body (1).
 2. The length measuring probe in accordance with claim 1, characterized in that the probe pin (2) is prestressed by means of two spring elements (7, 8), wherein an end area of one of the spring elements (7) is fixed in place on the probe pin (2), and the other end area of this spring element (7) is fixed in place on the guide element (4), and an end area of the further spring element (8) is fixed in place on the guide element (4), and the other end area of this spring element (8) on the base body (1).
 3. The length measuring probe in accordance with one of the preceding claims, characterized in that the guide element is a ball bearing guide (4).
 4. The length measuring probe in accordance with claim 3, characterized in that the ball bearing guide (4) consists of a ball bearing cage (41) and balls (42) rotatably seated in it, and that the end areas of the spring elements (7, 8) are fixed in place on the ball bearing cage (41) of the ball bearing (4).
 5. The length measuring probe in accordance with one of the preceding claims, characterized in that the probe pin (2) is seated, displaceable in the measuring direction X, by at least one further guide element (3) on the base body (1), on which none of the spring elements (7, 8) have been fixed in place.
 6. The length measuring probe in accordance with claim 5, characterized in that the further guide element is a ball bearing guide (3) between the probe pin (2) and the base body (1).
 7. The length measuring probe in accordance with claim 5, characterized in that the further guide element (3) is rigidly fixed in place on the base body (1).
 8. The length measuring probe in accordance with one of the preceding claims, characterized in that the spring elements are helical springs in the form of compression springs (7, 8).
 9. The length measuring probe in accordance with one of claims 1 to 7, characterized in that the spring elements are helical springs in the form of tension springs (10).
 10. The length measuring probe in accordance with one of the preceding claims, characterized in that the spring elements (7, 8) have the same spring properties, so that the guide element (4) is maintained centered between the spring elements (7, 8). 